Most coals inherently contain some non-combustible mineral matter (reported as the ash value of the coal) that exists in close association with the combustible carbonaceous solids. In order to improve the value of such coals, such as the combustion value, it is necessary to remove as much of this mineral matter as possible. This beneficiation can be accomplished by finely dividing the coal and separating combustible coal particles from mineral-containing particles. Froth flotation is a common method used to beneficiate finely-divided coals. Conventional techniques involve the passage of air through a suspension of the finely-divided coal to create finely disseminated air bubbles which creates a froth and preferentially carries the carbonaceous coal particles to the surface.
Since the surface of coal is generally hydrophobic, it is possible to preferentially float finely divided coal particles from finely divided mineral matter (recovered in the tails) in the presence of a frothing agent, such as methyl isobutyl carbinol. In this way the combustion value of the finely-divided coal can be improved. Unfortunately, many coals have experienced some degree of surface oxidation, such as oxidized bituminous coals, which reduces the hydrophobicity of their surface and interferes with their ability to float. As a result, in the absence of any treatment to improve surface hydrophobicity of such coals, the tail fraction from the flotation may contain a significant fraction of combustible material, thus reducing flotation yield.
Substances used to enhance surface hydrophobicity and improve the yield of a coal flotation are known as collectors. Collectors are generally surface active reagents which preferentially wet or adsorb on coal surfaces. Water insoluble, neutral hydrocarbon liquids derived from petroleum, wood, or coal tars have usually been employed in the froth flotation of coal. Historically, a number of fuel oils have been used as collectors, such as diesel oil, kerosene, furnace oil, Bunker C fuel oil, and mixtures thereof to enhance the surface hydrophobicity of the combustible coal particles. In this way, the yield of reduced ash coal may be significantly improved.
Recently, conventional coal beneficiation practices have come under heightened environmental scrutiny. It has long been the practice of coal treatment facilities to discharge the recovered tailings by landfill. Unfortunately, the most commonly used fuel oil collectors are not environmentally friendly and regulations and/or legislation limiting or possibly prohibiting their use is anticipated.
As a result, efforts are underway to find more environmentally acceptable materials that will function effectively as collectors, i.e., that promote the flotation of the combustible coal particles in preference to the non-combustible mineral particles, in the established processes of beneficiating coal by froth flotation.